The invention relates generally to breathing systems, and more particularly to a semi-closed circuit rebreather capable of supplying fresh make-up gas to a user in accordance with their activity level.
Breathing systems are used in a variety of underwater, fire fighting and hazardous material handling applications by the military, scientific and sporting communities. A variety of underwater diving applications are beginning to utilize semi-closed circuit rebreather systems in which fresh make-up gas (i.e., oxygen rich gas) is mixed with the user""s exhaled and recycled gas. The advantage of rebreather systems is that they provide for longer bottom times when compared to open circuit SCUBA. A conventional semi-closed circuit underwater breathing apparatus is illustrated in FIG. 1 and is referenced generally by numeral 10.
Apparatus 10 uses a controlled orifice 12 to provide a constant mass injection of fresh make-up gas from a supply 14 into a recycled gas breathing circuit 16. Briefly, recycled gas breathing circuit 16 includes a mouthbit 18 coupled to one of an inhalation bag 20 or an exhalation bag 22 as determined by check valves 24 and 26, respectively. A carbon dioxide scrubber 28 is coupled to bags 20 and 22. In operation, user exhalation causes check valve 24 to close and check valve 26 to open thereby allowing exhaled gas to flow through scrubber 28. During inhalation, check valve 24 opens while check valve 26 closes. Fresh make-up gas as well as gas exiting scrubber 28 are mixed in bag 20 prior to being inhaled by a diver via mouthbit 18. A continuous flow of a mixture of oxygen and nitrogen (or oxygen and helium in deeper applications) is set by orifice 12 to avoid the physiological symptoms of hypoxia and acute oxygen toxicity.
Compared with open circuit, demand-flow underwater breathing apparatus, these semi-closed circuit designs conserve the fresh make-up gas supply which must be carried by the diver. Additionally, the inert gas component in these designs provides the diver with the capability to make deeper excursions than would be possible with closed-circuit, pure oxygen rebreathers.
A disadvantage of this circuit design is that the injection rate for the fresh make-up gas must be set to satisfy the oxygen requirements based on the highest diver activity levels that might be achieved during the dive. Since these injection rates are not coupled with the diver""s actual activity level, and consequently his metabolic oxygen consumption rate or respiratory minute volume (RMV) as it is known, this circuit design can experience considerable fluctuations in both circuit oxygen partial pressures and inert gas pressures as the diver""s activity changes. The constant injection rate of the fresh make-up gas also creates a considerable risk for hypoxia at high diver metabolic levels in shallow water, or acute oxygen toxicity at low diver metabolic levels at greater depths. In addition, the wide fluctuations in circuit oxygen pressures require decompression schedules that must be tailored to the worse-case inert gas pressures. However, these schedules may be unnecessarily conservative, or even counter-productive, when the circuit inert gas pressures are lower.
Accordingly, it is an object of the present invention to provide a semi-closed circuit rebreather system that supplies quantities of fresh make-up gas in accordance with changes in a user""s respiratory minute volume.
Another object of the present invention is to provide an underwater semi-closed circuit rebreather system that minimizes risks for a diver experiencing a variety of activity levels during a dive.
Still another object of the present invention is to provide an underwater semi-closed circuit rebreather system that provides a constant oxygen volume fraction in the breathing circuit regardless of a diver""s activity level.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a semi-closed circuit rebreather system has a mouthbit insertable in a user""s mouth and a chamber coupled to the mouthbit. A vacuum pressure develops in the chamber as a breathing gas is drawn by the user from the chamber. A positive pressure develops in the chamber as an exhalation gas is expelled by the user into the chamber. First open circuit means are coupled to the chamber for supplying a mass of fresh make-up gas thereto based on pressure in the chamber. The mass of fresh make-up gas is zero when there is positive pressure in the chamber. The mass of fresh make-up gas increases as vacuum pressure in the chamber develops and increases. Second closed circuit means are coupled to the chamber for receiving and processing the exhalation gas to produce a recycled gas suitable for breathing. A volume of the recycled gas is supplied to the chamber based on pressure in the chamber. Specifically, the volume of recycled gas is zero when there is positive pressure in the chamber and when there are only low levels of vacuum pressure in the chamber, i.e., indicative of low RMV. Once a threshold vacuum pressure is reached, a volume of recycled gas is supplied to the chamber and increases proportionally to increases in the mass of fresh make-up gas as vacuum pressure increases beyond the threshold vacuum pressure. Thus, during higher levels of RMV, the recycled gas and fresh make-up gas mix in the chamber prior to inhalation therefrom.